Re: MTU

Yaseen,

mtu 1500 - Affects all packets entering and exiting the interface on a
router.

IP MTU - Affects only IP packets entering and exiting the interface

VLAN MTU - The maximum size allowed to traverse a VLAN. (The book
definition, although I dont think it works)
I set the MTU under the VLAN to 1000 mtu.

vlan 50
 name Customer2
 mtu 1000

SW1#sh vlan mtu

VLAN SVI_MTU MinMTU(port) MaxMTU(port) MTU_Mismatch
---- ------------- ---------------- --------------- ------------
1 1500 1500 1500 No
35 - 1500 1500 No
40 1500 1500 1500 No
****50 - 1500 1500 No*****
60 - 1500 1500 No
70 - 1500 1500 No
1002 - 1500 1500 No
1003 - 1500 1500 No
1004 - 1500 1500 No
1005 - 1500 1500 No

So I test with pings across the VLAN between to routers with the MTU set to
something above 1000 with the DF-BIT set and it works without an issue.
R1#ping 10.10.50.2 size 1500 df-bit

Type escape sequence to abort.
Sending 5, 1500-byte ICMP Echos to 10.10.50.2, timeout is 2 seconds:
Packet sent with the DF bit set
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/6/8 ms

If you try to set the ip mtu on a vlan SVI you get:
SW1(config-if)#ip mtu 1000
% ip mtu is not supported on this interface

The MTU can not be adjusted at the interface (switched or routed) with:
mtu
ip mtu

The only place I can see to adjust the MTU on a switch is with:
SW1(config)#system mtu ?
  <1500-1998> MTU size in bytes
  jumbo Set Jumbo MTU value for GigabitEthernet or TenGigabitEthernet
               interfaces
  routing Set the Routing MTU for the system

But...as you can see, you can only make the MTU bigger not smaller and the
routing can only be 1500.

Chris

On Tue, Aug 31, 2010 at 10:12 AM, Yaseen Mirza <yaseengs_at_gmail.com> wrote:

> Hi
>
> Can someone explain the difference between the following MTUs
>
> From what I can understand there is a hardware MTU.
>
> interface f1/0
> mtu 1500
>
> Then there is an IP MTU for only IP packets
>
> interface f1/0
> ip mtu 1500
>
> The most confusing is vlan MTUs
>
> Under what circumstance does the VLAN MTU come into play
> For instance
> if I change the MTU under a layer2 vlan what effect does it have and
> how is this different from changing the MTU under an SVI?
>
> Thanks
>
>
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Received on Tue Aug 31 2010 - 11:53:21 ART